On a printed circuit board of an electric/electronic device, separate elements, such as a ceramic oscillator, a crystal oscillator, a voltage controlled oscillator, a SAW filter, a diplexer, a coupler, a balun, an LPF, a BPF, and a dielectric duplexer; various modules each having a plurality of such elements integrated therein, such as an antenna switch module, a front end module, an RF integrated module, a radio communication module, an image sensor module, a tuner module, and a wireless LAN module; and parts such as a detection switch are mounted. These are used inside a metal case or covered with a cover, for electromagnetic shielding. In addition to this, as more electric/electronic devices become portable, the case is required to be thinner and smaller and its height is 5 mm for modules, and less than 2 mm or around 1 mm for separate elements.
A terminal connector at the printed circuit board side such as a mother board, a keyboard, or an LCD (Liquid Crystal Display) driver, and a connector at the side of an FPC cable require electromagnetic shielding property for the purpose of preventing communication noises and static electricity; and these are used as covered with a conductive metal case, cap, or cover. Similar to the above case, as devices become smaller and thinner, the connectors and sockets are made smaller and thinner.
However, in the case of the above-mentioned metal cases and the like, reduction of height is accompanied by reduction of internal volume; and there is a problem that insulation between the inner parts, terminals or wiring circuits and the electric/electronic part such as case, cover, cap or package (cover case) is not sufficiently assured.
For such instances, in the conventional art, an insulation film is cut into a sheet of predetermined dimensions and inserted into the case (see, for example, Patent Literature 1), or a metal material having a resin film formed on a metal base in advance is cut into predetermined dimensions (see, for example, Patent Literature 2). Use of a material having a resin film formed on a metal material in advance is preferable in view of economy or productivity as punching or bending working can be performed continuously. Besides, the material is such that it can be coated continuously and with high quality at a part, entire surface or double surfaces thereof as desired. Thus, recently, such materials tend to be used popularly.
With the advance of miniaturizing and sophistication of digital devices and portable devices, many restrictions are imposed on the shape of electric/electronic parts to be mounted thereon. Therefore, working into a desired shape becomes severe, and adhesion in various working process needs to be enhanced. As methods of enhancing adhesion between the metal base and resin film, the following methods can be mentioned, for example, a method wherein the metal base is coated at the surface with a coupling agent (see, for example, Patent Literature 3) and a method wherein a plated layer having a dendrite crystal is formed on a surface of the metal base (see, for example, Patent literature 4).
In addition, in forming a composite material of a metal base and a resin film, there are methods for preventing occurrence of curling in the manufacturing process; they are: a method, when coating a heat resistant resin solution continuously on a metal foil and drying the metal foil to be a flexible metal laminated material, rolling up the material with a predetermined amount or more of solvent left thereon, and subjecting the material to heat treatment while controlling solvent removal and crosslinking reaction of the resin, thereby to manufacture the flexible metal laminated material (see Patent Literature 5) or a method of producing a metal laminated plate having one or more polyimide-series resin layers and a metal conductor, in which the first polyimide-series resin layer in contact with the conductor is mainly composed of aromatic polybasic acid, its acid anhydride, diamine, and diisocyanate, and the residual solvent quantity in the first polyimide resin layer is 20 to 30% by mass (see, for example, Patent Literature 6).
Furthermore, it has been suggested to secure sufficient insulating property with an embedded connector itself by providing a resin film at a site on a metal base where insulation is needed, and, in order to secure sufficient heat-resistant mounting property by solder bonding, it has been suggested to prevent the occurrence of whiskers by forming an Sn-based plating layer having excellent solder wettability on the surface of the areas other than the resin film is formed, and allowing re-melting and solidifying (reflow) by a reflow treatment (see, for example, Patent Literature 7).
Here, when a composite material having an insulating film on a metal base is used as a material for the electric/electronic part, such a material is worked in the following manner. For example, as this material has an insulating film on the metal base, the material is subjected to working such as punching at a part containing a boundary face between the metal base and the insulating film, to form a connector contact or the like. This feature allows arranging such connector contacts at a narrow pitch, and various applications are possible. In addition, after working such as punching, the material may further be subjected to bending working thereby to be applicable to electric/electronic parts having various functions.
When this composite material is subjected to working such as punching at a part including a boundary face between the metal base and the insulating film, a small gap of several μm or several tens μm is sometimes formed at the worked part between the metal base and the insulating film. It is assumed that this gap is formed by insufficient adhesion between the metal base and the resin film. This state is schematically illustrated in FIG. 8. In FIG. 8, 10 denotes a electric/electronic part, 11 denotes a metal base, 12 denotes an insulating film, and a gap 13 is formed between the metal base 11 and the insulating film 12 in the vicinity of a punched surface 11a of the metal base 11. This tendency becomes strong as the clearance in punching becomes larger (for example, 5% or more relative to the thickness of the metal base). As there is a practical limitation for reducing of the clearance in punching, it can be said that this tendency becomes increased as the above-mentioned worked piece is more miniaturized.
In such a state, due to secular changes in punched portion or the like, the insulating film 12 becomes completely peeled off from the metal base 11, and provision of the insulating film 12 on the metal base 11 becomes meaningless. Besides, providing of the insulating film after fine working needs much time and effort and brings about cost increases, which is not practical. Further, if a metal exposed surface of the electric/electronic part formed (for example, punched surface 11a) is to be used as a connector contact or the like, a metal layer may be later formed on the metal exposed surface (for example, punched surface 11a) by plating. However, when such a part is immersed in a plating liquid, the plating liquid may infiltrate via the gap 13 and promote peeling off of the insulating film 12 from the metal base 11.
Besides, when bending is performed after punching, even if no gap is formed between the metal base and the insulating film at a worked part in the process of punching or the like, a gap is sometimes formed between the metal base and the insulating film after bending is performed. This is schematically illustrated in FIG. 9. In FIG. 9, 20 denotes an electric/electronic part, 21 denotes a metal base, 22 denotes an insulating film, and a gap 23 is formed at an inner side of a bent part of the metal base 21 and a gap 23, 24 is formed at an end (particularly at an outer side of the bent part) of the electric/electronic part 20. As illustrated in FIG. 9, these gaps 23 and 24 are prominently formed at the inner surface side and side surface of the bent part of the bent electric/electronic part and at the end of the electric/electronic part, and they may cause peeling off of the insulating film 22 from the metal base 21.    Patent Literature 1: JP-A-1-6389 (“JP-A” means unexamined published Japanese patent application)    Patent Literature 2: JP-A-2004-197224    Patent Literature 3: Japanese Patent No. 2802402    Patent Literature 4: JP-A-5-245432    Patent Literature 5: JP-A-2001-105530    Patent Literature 6: JP-A-2005-117058    Patent Literature 7: JP-A-2006-86513